The present invention relates to manufacture of patterned resistive sheets and the resulting article of manufacture.
Resistive sheets have been used for attenuating, shielding and modulating electromagnetic fields, microwaves fields, and so forth, generated in the vicinity of other electrically powered components. A conventional resistive sheets or films basically involves a thin pattern of metal attached to thin, flexible polymeric substrate film where the metal pattern is designed to yield a desired ohms per square characteristic across the surface of the resistive sheet when measured with a four point probe. The polymeric substrate typically is a thin material capable of flexure out of its major plane to permit deformation of the resistive sheet accommodate any a three-dimensional installation that might be envisaged. The formation of the preselected pattern of metal on the polymeric substrate can be executed via an additive process or a subtractive process.
For instance, manufacturers of two-sided resistive metallized sheets have used a number of approaches to create a desired pattern in the surface of a metallized polymeric sheet or film.
In one previous subtractive approach summarized in the flow diagram shown in FIG. 1, a first step A involves coating each metallized surface of the polymeric substrate with a photosensitive polymer composition, i.e., a resist. Alternately, the metallized surface of the polymeric substrate has been laminated with two preformed solid sheets of photosensitive resist using a two roll mill by feeding the three requisite sheets in the requisite lay-up into the nip of the compression rolls. However, a drawback associated with laminating the resist to the substrate is that the preformed, solid resist sheets tend to wrinkle and incur other surface distortions during lamination to the metallized surface, which impact the resolution of the subsequent etch performed on the exposed metal surface. Accordingly, the following discussion describes the process flow in the context of using a resist coating for any thickness of metallized polymeric substrate.
In step B indicated in FIG. 1, the resist-coated substrate is soft baked to remove solvent from the resist coating. Then, in step C indicated in FIG. 1, an imager with two reticles or master masks defining the desired exposure patterns are aligned in a coordinated manner over the opposing major surfaces to ensure patterns in registration or intentional offset are provided. The reticles are imaged directly on to the resists with U.V. light to create the specified exposure patterns as between the top and bottom metallized surfaces. The imager uses a source of light at a specific wavelength that will crosslink those regions of the resist polymer that are exposed to light projected through the reticle to delineate certain exposed and non-exposed patterns in the resist. The crosslinked (exposed) resist portions later serve as a temporary mask for a subsequent step of chemically etching the exposed metallized surfaces during photofabrication. However, initially after radiation exposure, the resist is subjected to post-exposure baking (i.e., a hard bake) to advance crosslinking in the exposed areas and densify these areas, as indicated in step D of FIG. 1.
Next, as indicated by step E of FIG. 1, the non-irradiated (non-exposed) resist regions are developed, e.g., in an aqueous sodium carbonate solution, which dissolves the non-exposed resist portions, thereby forming openings in the resist coating through which surface regions of the underlying metal surface are exposed that correspond to the non-exposed resist regions.
Subsequent to development and in a separate operation indicated as Step F in FIG. 1, the metal regions exposed in the openings formed in the resist in the prior development step E are removed with a different wet chemical etchant than the developer solution. For step F, the etch selectivity for this metal etch is manipulated such that the etchant used removes the exposed metal much more rapidly than the crosslinked (exposed) resist regions so as to define a discontinuous metallized surface in a desired pattern having a desired measurable resistance associated with it.
Finally, in step G indicated in FIG. 1, the temporary crosslinked resist covering the non-etched metal regions is stripped with another chemical, such as a caustic or amine solution. The metal film disposed under the resist has to be corrosion resistant to the stripper solution. In the case of aluminized film, the conventional caustic stripper tends to be overly aggressive resulting in undercutting of portions of the metal pattern. Additionally, since conventional negative working polymeric photoresists tend to be based on polymer chemistry which degrades if exposed to in-service temperatures of about 140-150.degree. F. (60-66.degree. C.), so these types of resists must be removed (stripped) from the resistive sheet if workpiece operation temperatures of above 100-110.degree. F. (38-43.degree. C.) are anticipated.
Where a sheet of MYLAR.RTM. or KAPTON.RTM.. Which has been aluminized on both sides, is used as the material to make the resistive sheet, the aluminum tends to be corroded during the last step, Step G, involving stripping the resist off the metal pattern. This situation has complicated the use of aluminum as a low cost material for resistive sheets since the aluminum tends to be less tolerant of aggressive resist strippers. The aluminum patterns may withstand an amine solution used for resist stripping. However, the conventional amine chemicals used for resist stripping. e.g., ethylene diamine, generally are relatively expensive and require special precautions during storage, use, and disposal that most operators want to avoid.
More generally, as another mode of pattern transfer processing for defining a metal pattern on a polymeric substrate, U.S. Pat. No. 4,869,778 describes a process for the microdemetallization of an aluminized MYLAR.RTM. film involving printing a micropattern of a caustic resistant U.V. curable resin on the aluminum surface, then etching of the exposed aluminum regions with warm saturated caustic solution, followed by immediate rinsing of the patterned surface with an acidic solution to neutralize the etchant. The caustic resistant resin resides on the metal pattern formed upon completion of the process. The micropatterns defined by the process of the '778 patent are stated to involve aluminum line widths of from about 0.2 to 2 mils. The use of printed resists has the drawback that very high resolution, fine minimum dimensions in metal patterns may not be possible.